1. Field of the Invention
This invention relates to SRAM memory devices and more particularly to improved load resistors therefore.
2. Description of Related Art
U.S. Pat. No. 5,200,356 of Tanaka for "Method of Forming a Static Random Access Memory Device" and U.S. Pat. No. 4,950,620 of Harrington for "Process for Making Integrated Circuit with Doped Silicon Dioxide Load Elements" show SRAM devices with load resistors formed by ion implantation of impurities in a silicon dioxide film.
U.S. Pat. No. 4,717,681 of Curran for "Method of Making a HeteroJunction Bipolar Transistor with SIPOS" shows a method for manufacturing a semiconductor device which uses a Semi-Insulating Polycrystalline Silicon (SIPOS) as described at Col. 6, lines 50-63, etc. where Curran states that SIPOS films are deposited conventionally at low temperatures (620.degree. C.) by LPCVD (low pressure chemical vapor deposition) of silane and nitrous oxide. The films may be deposited either doped or undoped, by in-situ inclusion of a doping gas such as phosphine. Alternatively, SIPOS films can be fabricated by molecular beam deposition with simultaneous electron beam evaporation of silicon and ion implantation of oxygen and ion implantation of dopants, such as phosphorus, if desired. The dopants can be electrically activated by means of rapid thermal annealing (RTA).
Sze "VLSI Technology" page 248 "The addition of oxygen to polysilicon increases the film resistivity. The resulting material, semi-insulating polysilicon (SIPOS), is used as a passivating coating for high voltage devices . . . SIPOS is deposited when silane reacts with small amounts of nitrous oxide at temperatures between 600.degree. and 700.degree. C. The two simultaneous reactions are EQU SiH.sub.4 .fwdarw.Si+2H.sub.2 EQU SiH.sub.4 +xN.sub.2 O.fwdarw.SiO.sub.x +2H.sub.2 +xN.sub.2
The quantity of nitrous oxide in the reaction determines the film composition and resistivity . . . SIPOS has a multi-phase microstructure containing crystalline silicon, amorphous silicon and silicon monoxide. The specific composition depends on the deposition temperature, the amount of nitrous oxide used in the reaction, and the time and temperature of post-deposition anneals. SIPOS used in VLSI circuits contains between 25 at. % (atomic percent) and 40 at. % oxygen.
FIG. 2 is a sectional view of a fragment of a prior art SRAM (Static Random Access Memory) device 9 with a substrate 10 composed of silicon. The device includes a FOX (field oxide) region 11 upon which a polysilicon 1 gates 12 and 16 are formed with silicon dioxide spacers 13 on either side. Polysilicon 1 gate 16 extends across the edge of the FOX region 11 in direct contact with the surface of the silicon substrate 10. Above that structure is formed a dielectric layer 14 (comprising an interpolysilicon (IPO) structure) composed typically of SiO2 with an opening 17 therethrough above the gate 16. Next, a blanket layer 15 of undoped polysilicon 2 resistor material is formed over the preceding layers. Layer 15 in stacked contact through opening 17 with polysilicon 1 gate 16.
Prior art polysilicon ion implanted resistors have had a length of about 2.6 .mu.m with a width of about 0.4 .mu.m to about 0.6 .mu.m.
The problem with conventional polysilicon load resistors is that they suffer large resistance variations and high temperature coefficients.